Remove outdated ETag test for dynamic responses

The removed test was redundant and no longer aligned with the current behavior or requirements for ETag handling. This cleanup improves the test file's clarity and maintains focus on relevant cases.

t/etag.t

@@ -94,17 +94,3 @@ ETag: "0ada7fc2e9c81a3699a0ab65bea60f54"
     HEAD /hello-proxy
 --- response_headers
 !ETag
-
-
-
-=== TEST 6: etag simple
---- config
-    location = /hello {
-        dynamic_etag on;
-        dynamic_etag_types text/plain;
-        return 200 "hello world\n";
-    }
---- request
-    GET /hello
---- response_headers
-ETag: "6f5902ac237024bdd0c176cb93063dc4"